I'd like to downgrade my plan type (ex. Business to Team, Business to Developer, or Team to Developer) but it's not clear to me when will the changes be applied.

Paid plans are purchased for the current monthly period and cannot be downgraded mid-month. Downgrades are processed at the end of the current billing cycle and will apply to the following period.
​
Downgrading your plan type can be done from the Subscription Management flow https://sentry.io/orgredirect/settings/billing/checkout/, specifically: Settings > Subscription > Manage Plan. Only members with the Owner or Billing role have permission to make changes to the plan type.